The present invention relates to a method for the manufacture of diaphragms of fibrous material and an organic binder for electrochemical cells, as well as to diaphragms made according to such a method.
Diaphragms for electrochemical cells, in addition to possessing sufficient porosity and good chemical resistance to the media used, also must have high mechanical strength. Accordingly, it is known to use in fuel cells, for example, diaphragms comprised of high-purity asbestos fibers and a plastic material, in which the plastic material is a binder of methacrylic-acid ester, polysulfones, chlorosulfonated polyethylene or chloroprene in amounts of 0.5 to 6% by weight, particularly 1.5 to 3% by weight, referred to the weight of the asbestos fibers (British Pat. No. 1,213,472). In the preparation of the asbestos diaphragms, the plastic is added to the asbestos fibers in the form of an aqueous dispersion.
U.S. Pat. No. 3,583,891 describes a method for the manufacture of a gas-tight diaphragm of high mechanical strength for use in electrochemical cells, particularly fuel cells. The diaphragm consists of fibrous asbestos or carbon material and a plastic binder, in which a butadiene-styreneacrylonitrile copolymerizate is added to the fiber material when the diaphragm of the latter is made (copolymerizate content of 6 to 15% by weight as referred to the fiber material), and where, after the diaphragm is formed, the nitrile groups of the copolymerizate are saponified into carboxyl groups.
The requirements for the diaphragms with respect to their chemical and mechanical stability become more stringent when higher operating temperatures are employed, which is due to oxygen and electrolytic liquid. An additional aggravating circumstance is the relatively high operating gas pressure.
Asbestos diaphragms have found acceptance, for example, in H.sub.2 /O.sub.2 fuel cells with an alkaline electrolyte. However, at operating temperatures above 80.degree. C., the asbestos material gives off substantial quantities of silicates to the electrolytic liquid, which lower the activity of the Raney nickel used as the anode catalyst. In addition, the diaphragms suffer a loss of mechanical strength due to the attack by the electrolytic liquid. This can be compensated only in part through the use of thick diaphragms having a high binder content. Additionally these diaphragms possess other detrimental properties.